The present invention relates to a hydraulic control apparatus for a belt-drive CVT (continuously variable transmission) of a vehicle, which continuously changes a transmission ratio by way of controlling a separation between the two sides of each of the drive and driven pulleys by working fluid pressure.
The belt-drive CVT is provided with a primary pulley (drive side pulley), a secondary pulley (driven side pulley) and a belt (V-belt) wound around the both primary and secondary pulleys. Each of the primary and secondary pulleys is formed from a stationary sheave (pulley) which is integrally formed with a shaft (a primary shaft or a secondary shaft) as an axis of rotation, and a movable sheave (a slide pulley) that is movable in an axial direction of the shaft. These stationary and movable sheaves are coaxially aligned with each other, and facing to each other. And the belt is pressed between a V-shaped groove formed by the stationary and movable sheaves, then power is transmitted from the primary pulley to the secondary pulley.
Each of the movable sheaves of the primary and secondary pulleys is movable in the axial direction of the shaft so that the movable sheave separates from the stationary sheave or moves closer to the stationary sheave, by controlling fluid pressure in a fluid pressure chamber formed behind the movable sheave. Further, by way of this movement (stroke displacement) of the movable sheave, a width of the groove is adjusted. Thus, effective radius of rotation of the pulley is adjusted, and therefore a power transmission ratio from the drive side pulley to the driven side pulley is continuously varied.
For instance, when setting the transmission ratio to small (namely, high speed), the groove width of the drive side pulley is decreased by increasing the fluid pressure in the fluid pressure chamber of the drive side pulley and by pushing the movable sheave toward the stationary sheave. And the effective radius of rotation of the belt running around the drive side pulley becomes large. At this time, as a matter of course, since a length of the belt does not change, as the radius of rotation of the drive side pulley becomes large, the radius of rotation of the driven side pulley becomes small. And therefore, the transmission ratio can be set to small.
In the above belt-drive CVT for the vehicle, there may be the following drawback when changing the transmission ratio to maximum (namely, lowest speed) or to minimum (namely, highest speed or top speed). FIG. 5 is an example showing variations in the fluid pressure of the drive and driven side pulleys and the transmission ratio with time in a case where the transmission ratio is set to minimum (highest speed). When an operation of the speed change starts, the working fluid pressure is provided for the fluid pressure chamber of the drive side pulley, and the fluid pressure of the drive side pulley rises rapidly (at T1). Then, after the fluid pressure of the fluid pressure chamber has reached a predetermined fluid pressure, the movable sheave of the drive side pulley is pushed toward the stationary sheave, and the stroke displacement of the drive side pulley is initiated (at T2). As the groove width of the drive side pulley is decreased and the effective rotation radius becomes large with the displacement of the movable sheave, the transmission ratio gradually continuously becomes smaller and finally the highest speed transmission is obtained. Here, as the movable sheave starts moving toward the stationary sheave, the fluid pressure chamber of the drive side pulley is provided with the working fluid by an amount corresponding to an amount of the displacement of the movable sheave. However, at this time, a volume of the fluid pressure chamber increases by a volume corresponding to the amount of the displacement of the movable sheave. As a result of this, the fluid pressure of the drive side pulley increases moderately from T2. After that, when the transmission ratio has reached the minimum ratio (when the highest speed transmission has been obtained) at T3, the movable sheave of the drive side pulley is suddenly stopped by mechanical movement limitation. On the other hand, the working fluid continues flowing into the fluid pressure chamber by inertia. In other words, the flow of the working fluid into fluid pressure chamber does not stop rapidly due to the inertia, and can not respond quickly to the sudden stop of the movable sheave. Accordingly, even though the movable sheave stops due to the mechanical movement limitation and the increase of the volume of fluid pressure chamber also stops, the working fluid continues flowing into the fluid pressure chamber for a while. Therefore, a sharp or sudden increase in fluid pressure occurs at the drive side pulley, and an overshoot of pressure arises.
Meanwhile, as for the driven side pulley, although the movable sheave suddenly stops by mechanical movement limitation in the same manner as the drive side pulley, the working fluid continues flowing out of the fluid pressure chamber of the driven side pulley because of inertia. As a result of this, a sharp or sudden decrease in fluid pressure occurs at the driven side pulley, and an undershoot of pressure arises.
As described above, a phenomenon in which the fluid pressure temporarily rapidly changes is called “a surge pressure”. And the surge pressure occurs in a case as well where the transmission ratio is set to maximum (lowest speed) in the same manner as set to minimum (highest speed). In the case of setting of the lowest speed transmission, the fluid pressure chamber of the driven -side pulley is provided with high fluid pressure. And the movable sheave of the driven side pulley is pushed toward the stationary sheave, then the transmission ratio varies by way of changing of effective rotation radius of the driven side pulley. Here, when the transmission ratio has reached the maximum ratio (when the lowest speed transmission has been obtained), the movable sheave of the driven side pulley suddenly stops. However, the flow of the working fluid into fluid pressure chamber of the driven side pulley does not stop rapidly, and can not respond quickly to the sudden stop of the movable sheave. The overshoot therefore arises at the driven side pulley by an excess surge pressure. Meanwhile, as for the drive side pulley, since an outflow of the working fluid can not respond quickly to the sudden stop of the movable sheave, the undershoot arises at the drive side pulley by the surge pressure.
In the above belt-drive CVT, when the excess surge pressure arises, there are possibilities that a load or shock resulting from the surge pressure may cause damage to the bet or reduction in life of the belt. In addition, when the undershoot of the fluid pressure arises at the drive or driven side pulleys, a grasping force which grasps or presses the belt by the fluid pressure might reduce, and there may arise a belt slip causing a pulley racing.
For the above problems, as a means of controlling of such surge pressure, Japanese Patent Provisional Publication No. 5-131295 (hereinafter is referred to as “JP5-131295”) shows an apparatus which reduces the surge pressure. In JP5-131295, an opening of a servo valve that regulates fluid pressure is feedback controlled (PID controlled). And the surge pressure is reduced by compensating for or correcting a control gain.